Photographic plate



United States Patent 3,280,734 PHOTOGRAPHIC PLATE Howard A. Fromson, Rogues Ridge Road, Weston, Conn. No Drawing. Filed Get. 29, 1963, Ser. No. 319,685 The portion of the term of the patent subsequent to May 4, 1982, has been disclaimed 17 Claims. (Cl. Nil-149.2)

The present invention relates to a photographic plate for use, for example, in lithographic printing or in the manufacture of a photographically printed plate for display or other purposes.

Photographic printing plates are known made by applying to an aluminum surface of an aluminum sheet an aqueous solution of an alkali metal silicate, such as sodium silicate, to form a water insoluble layer, which results from the reaction of said silicate with the aluminum, and which presents a hydrophilic, organophobic (ink repelling) surface, and applying to said layer a coating of light-sensitive water-soluble diazo resin. Upon exposure of the plate to light through a stencil or negative, this diazo resin will react in the exposed portions to form an image of water-insoluble, hydrophobic and organophilic material bonded to the hydrophilic silicate layer. After exposure, the parts of the diazo resin coating, which have not been exposed to light, and which are still watersoluble, are completely washed away, leaving the parts of the hydrophilic, organophobic silicate layer in the nonimage areas exposed. In the printing press, upon application of water and then printing ink to the plate constructed as described, the exposed hydrophilic, organophobic silicate layer will absorb water and reject ink and the image-forming diazo resin coating will be receptive to the ink and will reject the water. The plate can thereby be employed for planographic or lithographic printing in the press.

Aluminum surfaces do not have comparatively high resistance to abrasion or corrosion.

One object of the present invention is to provide a photographic plate which is superior to those known in the art, and to a method of making the same.

Another object of the present invention is to provide a new and improved photographic plate, which is highly resistant to abrasion and corrosion, and which has a layer serving as a photo-sensitive medium highly adherent to the surface of said plate.

As an important feature of the present invention, in the production of the photographic plate, instead of employing a sheet free from aluminum oxide coatings as previously proposed in the prior art, there is employed an aluminum base sheet having an aluminum oxide coating thereon produced, for example, by anodizing the aluminum sheet to form a porous anodic coating thereon consisting of aluminum oxide. Such an aluminum oxide surface is highly resistant to abrasion and corrosion and has the desired degree of hydrophilicy and organophobicy to make it suitable for selective inking.

An unsealed aluminum oxide coating on an aluminum base sheet formed in a certain manner artificially, as for example, by anodic treatment, as distinguished from one formed naturally, as for example, by mere exposure to the atmosphere is porous and should, theoretically afford good anchorage for a layer of light-sensitive material. However, it has been found that a layer of light-sensitive material, directly applied to an unsealed aluminum oxide coating, does not have sufficient continuity or uniformity to form good solid images throughout the image areas and disintegrates readily when subjected to abrasive action resulting from normal use. It is believed that the reason for this deficiency in the light-sensitive material is that the light-sensitive material seeps into the pores of the aluminum oxide coating and that the pores are of varying depths and other dimensions in different sections of the coating, thereby causing corresponding variations in the effective thickness of the light-sensitive layer. In any case, it has been found in accordance with the present invention, that if a substratum is formed in the pores of the unsealed aluminum oxide coating, and the light-sensitive layer is applied to the aluminum oxide coating containing such a substratum, the light-sensitive layer will be strongly adherent, and will form good image areas of uniform integrity when exposed and developed and will Withstand effectively abrasion and disintegration during normal use. It is believed that this substratum not only reduces excessive seeping of the light-sensitive material into the pores, but serves to eliminate defects due to irregularity in the dimentions of the pores in the different sections of the aluminum oxide coating, since this substratum affords a substantial uniform base for the light-sensitive layer.

In accordance with certain aspects of the invention to an unsealed porous oxidized surface of an aluminum plate, attained desirably by anodizing the surface of said plate, there is applied a solution, which is fluid enough and which has its molecules of dissolved solutes small enough to enter the pores of the anodized surface and to serve as a filler for said pores, and as a substratum for the photosensitive material to be subsequently applied after the major portion of the solvent has been driven off. The solvent should desirably be one that can be evaporated. Also, the solution or the solute must be such as not to form a solid precipitate in the pores which would entirely block the pores. Moreover, the solution or the solute must be such as not to produce a hydrophobic or organophilic surface.

After the surface of the aluminum oxide is treated as described and is still unsealed, it is wiped clean or rinsed and dried. Under these conditions, the filler solute or substrate formed therefrom, although deposited into the pores of the aluminum oxide surface, still leaves sufficient spaces in the pores of the oxide surface to permit the light-sensitive material subsequently applied to the treated surface to seep into these spaces to afford the necessary anchorage to the layer of light-sensitive material formed.

On the surface treated as described, the light-sensitive material is applied in the usual manner, as for example, by rolling, coating, sponging, curtain coating, spraying or dipping.

There may be cases where the substrate material deposited in the pores of the aluminum oxide surface completely fills these pores, but if this material itself is porous and is hydrophilic and organophobic, then this material will serve the purpose of the present invention. In that case, there are sufficient spaces left in the orginial pores of the aluminum oxide surface through the pores of the deposited substrate material in said original pores to cause the light-sensitive material employed to enter into these pore spaces.

In accordance with the present invention, it has been found that such water-soluble materials as ammonium bichromate, sodium bichromate, potassium bichromate, ferric ammonium oxalate, and any of the water-soluble dyes that will dye aluminum oxide surfaces, deposited on the oxide surface before the application of the light-sensitive material, will, when the solvent is substantially driven off, be suitable as pore filler materials and as substrates for the purpose of the present invention.

In the process of making the photographic plate of the present invention, an oxide coating is formed on the surface of an aluminum base sheet in any manner well known in the art. By aluminum base sheet is meant a sheet, which aside from its oxide coating contains essentially aluminum in uncombined form and which may contain alloying substances. If the aluminum sheet is to be em- Patented Oct. 25, 1966' ployed in the making of a lithographic printing plate, the sheet may have a thickness of, for example, between .005 inch and .015 inch and the aluminum oxide surface formed thereon should preferably be less than .0005 inch thick to prevent crazes or fissures in the printing plate when applied to the printing roller of a printing press. The pores in such an aluminum oxide coating may range between 50 and 400 Angstroms in diameter.

The aluminum oxide surface may be formed by anodizing and for that purpose, the aluminum sheet may be anodized at 70 to 100 F. by immersion in an electrolyte comprising between and 25% by weight of sulfuric acid in water solution for a period of one to fifteen minutes, while applying a voltage of approximately 12 to 18 volts.

The aluminum base sheet having the unsealed porous aluminum oxide coating is coated with a solution of the pore filler material of the type described, desirably by dipping the base sheet in said solution, the solution being thin enough to seep freely into the pores of the coating and to deposit when dried sufficient filler material to serve the substrate function described.

The aluminum sheet treated as described is then rinsed and dried to produce a photo-sensitized plate.

Where water-soluble diazo resin is desired as the lightsensitive material, this resin is easily available commercially or may be manufactured as described in US. Patent No. 2,714,066 and its concentration and manner of application in the process of making the photographic plate may be as indicated in said patent. After the plate with the diazo resin coating is exposed to suitable light, as for example, ultra-violet light or other actinic light, for the required period, as for example, from 1 to 5 minutes, through a stencil or negative, it may be developed by wiping with a suitable desensitizing or developing agent, well-known in the art. The desensitizing agent may be a water-solution of a gum, such as gum arabic and serves to dissolve and remove the diazo resin not light-reacted and not insolubilized. The gum in this solution enhances and reinforces the hydrophilicy of the aluminum oxide surface in the non-image areas. The aluminum oxide surface is especially receptive to the gum in the solution and holds it firmly. This gum solution may also contain a developing ink to coat the image areas therewith so that the image areas and non-image areas may be more sharply contrasted and defined.

If it is desirable to reinforce or enhance the hydrophilicy of the aluminum oxide surface containing a substratum, and if the photo-sensitive material is such as to be affected adversely by gum, then some other suitable type of developing or desensitizing agent other than a gum solution may be employed. For example, a glycerine base desensitizer, consisting essentially of a water solution of glycerine may be employed.

Where the light-sensitive material is an aqueous solution of film-forming polyvinyl, containing a light-sensitizing agent, such as polyvinyl alcohol, this polyvinyl si highly hydrolyzed to cause it to go easily into solution. The concentration of polyvinyl alcohol, for example, in solution is desirably from 8 to by weight. Added to this polyvinyl alcohol solution is ammonium bichromate desirable in an amount of /2 to 1 /2 by weight of the entire mixture of polyvinyl alcohol, water and ammonium bichromate. In place of the ammonium bichromate, equivalent amounts of the sodium, potassium or cupric bichromate may be used. The mixture of polyvnyl alicohol soluton and bichromate has the consistency of a light gum and can be applied in any suitable manner at substantially room temperature, as for example, by a roller coating method or by wiping with a soaked sponge or cheesecloth, or by any other suitable way known in the art. The resulting coating over the pore-filled aluminum oxide surface is dried, preferably by a stream of warm air at a temperature between 80100 F. By porefilled aluminum oxide surface is meant pore-filled to the extent indicteda above to permit the formation of residual spaces in said pores for the light-sensitive material to enter.

To develop the sensitized plate formed as described, the plate with the substrate layer of filler material in the pores of the aluminum oxide and the coating of lightsensitive material over said layer, is exposed to actinic light, such as ultra-violet light, through a stencil or negative for a short period of time suflicient to insolubilize the light-exposed portion of said light-sensitive coating, as for example, for a period of 1 to 5 minutes, depending on the intensity of the light, and the thickness of the coating. The light exposed portions of this light-sensitive coating will react to form an mage of water-insoluble, hydrophobic and organophilic material firmly bonded to the aluminum oxide coating and/orto the filler material in the pores of said coating, while the portions of the light-sensitive coating not exposed to the ultra-violet light remain unreacted and water-soluble. These unreacted and water-soluble portions of the light-sensitive coating are removed, as for example, by washing with water, with gum arabic solution, or with any other suitable developing or densitizing agent, to define the non-image areas of the printing plate and to expose in these areas, the hydrophilic, organophobic surface.

Whether diazo resin, polyvinyl alchohol-ammonium bichromate mixture, or any other suitable light-sensitive materal is employed for the light-sensitized layer, the printing plate is characterized by maximum water receptivity and ink repellency in the non-image areas, and because of the fact that a sheet having an aluminum base and an aluminum oxide coating thereon has been employed in the manufacture of the printing plate, the printing surface has extreme hardness, having a Mohs hardness of about 7, is highly resistant to scratching or abrasion, and has non-scumming characteristics. Because of the fine grain on the printing plate due to the aluminum oxide coating on the sheet employed in the making of the sheet, there is added fineness and fidelity in the reproductions printed by the plate.

Moreover, the light-sensitive layer will firmly adhere to the aluminum oxide coating and/ or substrate pore filler and will have sufiicient body to maintain its integrity against normal abrasive use for a long time, and will have an effective thickness substantially uniform throughout its area.

Example 1 An aluminum sheet having a thickness of about 0.005 inch is degreased by a grease solvent which may consist essentially of a chlorinated hydrocarbon or an alkaline cleaner, such as a solution of sodium hydroxide consisting of 6 oz. of hydroxide per gallon of water. The degreased sheet after rinsing with water is then anodized at F. in an electrolyte comprising 15 H 80 at a voltage of 15 volts for a period of about 8 minutes, to produce a sheet having an aluminum base and a coating of aluminum oxide.

The anodized aluminum sheet is then dipped in a strong solution of 510% water-soluble ammonium bichromate solution at a temperature of 212 F. for 15 seconds to 10 minutes under conditions, which will not cause sealing, as for example, by maintaining the pH of the solution below 4. More specifically, the ammonium bichromate solute is of 7% strength, the temperature is F. and the period of dipping is for 3 minutes.

The plate so prepared is then rinsed in cold water and dried and diazo resin is applied thereto. The diazo resin is of the type and is applied in the manner described in the aforesaid Patent 2,714,066 and the plate sensitized.

Exposure of the photographic plate may be carried out under a source of ultra-violet light through a negative or stencil for 2 minutes. After such exposure, the plate is developed by wiping with a solution of gum arabic to dissolve and remove the diazo resin not light-reacted and not insolubilized and to enhance the hydrophilicy of the non-image areas of the plate, if that is desired.

Example 2 The aluminum plate anodized as described in Example 1, is dipped in a water-solution of ferric ammonium oxalate containing -50 grams per liter of this solute and having a temperature of 120-180 F., for 10 seconds to 10 minutes. More specifically, the solution has 30 grams per liter of ferric ammonium oxalate, and has a temperature of 150 F., and a pH of 4.8 to 5.5 and the plate is dipped in this solution for 5 minutes. The ferric ammonium oxalate is normally not a dye, and before application to the anodized aluminum surface is a pale green solution. When applied to the unsealed anodized aluminum surface at the pH indicated, the ferric ammonium oxalate enters the pores of the anodized aluminum surface, is oxidized and is transformed into a golden precipitate. The precipitate will coat the walls of the pores of the aluminum oxide coating without entirely blocking these pores.

The plate so prepared is then rinsed in cold water and dried and the diazo resin is applied in the manner described in Example 1. The sensitized plate so produced may be developed in the manner described in Example 1.

Example 3 The aluminum plate anodized as described in Example 1, is dipped in a water-solution of Aluminum Blue LLW sold by Durand-Huguenin; this is a dye usually employed for dyeing anodized aluminum surfaces, and is chemically known as CI Mordant Blue 69. The concentration of this solution is 0.1 to 4 grams per liter of water, and specifically is about 2 grams per liter and its pH is from 3 to 4 /2. The temperature of the solution is from l-180 F., and more specifically from 140-150 F. and the time of immersion is 1 minute to 10 minutes, and specifically 3 minutes. The CI Mordant Blue 69 being a blue dye will coat the walls of the pores of the aluminum oxide coating with a blue precipitate without entirely blocking these pores.

The plate so prepared is then rinsed in cold water and dried and the diazo resin is applied in the manner described in Example 1. The sensitized plate so produced may be developed in the manner described in Example 1.

Example 4 The aluminum plate anodized as described in Example 1, is dipped in a water-solution of Golden Orange RIW sold by Durand-Huguenin; this is a dye usually employed for dyeing anodized aluminum surfaces, and is chemically known as CI Mordant Yellow 59. The concentration of this solution is 0.1 to 4 grams per liter of water and specifically is about 2 grams per liter and its pH is from 6 to 8. The temperature of the solution is from 120-180 F. and more specifically from 140- 150 F. and the time of immersion is 1 minute to 10 minutes, and specifically 3 minutes. The CI Mordant Yellow 59 being a yellow dye will coat the walls of the pores of the aluminum oxide coating with a yellow precipitate without entirely blocking these pores.

The plate so prepared is then rinsed in cold water and dried and the diazo resin is applied in the manner described in Example 1. The sensitized plate so produced may be developed in the manner described in Example 1.

Example 5 The anodized aluminum plate treated in the manner described in Examples 1 to 4 just up to the point where it is treated with diazo resin, is treated instead with a solution containing 20 parts by weight of polyvinyl alcohol 85 to 90% hydrolyzed (DuPont Elvanol 52-22), 128 parts by weight of water, and 8 /2 parts by weight of an 18% aqueous solution of ammonium bichromate. This mixture is applied with a sponge in the form of a thin coating. The plate so treated is then dried by a stream of warm air at about F. and constitutes a sensitized photographic plate. The sensitized plate is developed with a desensitizing agent consisting essentially of a glycerine base water solution.

Example 6 The anodized aluminum plate treated in the manner described in Examples 1 to 4 just up to the point where it is treated with diazo resin, is treated instead with a mixture containing 11 parts by weight of a polyvinyl acetate water emulsion (DuPoint Elvacet 81-900), 1 part by weight of polyvinyl alcohol in solid powder form (Du- Pont Elvanol 52-22), 4 parts by weight of water, and 7 parts by weight of 18% aqueous solution of ammonium bichromate. The treatment thereafter including the development of the photographic plate is as described in Example 5 Example 7 The anodized aluminum plate treated in the manner described in Examples 1 to 4 just up to the point where it is treated with diazo resin, is treated instead with a mixture containing parts by weight of polyvinyl acetate emulsion containing 55% solid and the rest water (DuPont Elvacet 81-900) and 50 parts by weight of 18% aqueous solution of ammonium bichromate. The treatment thereafter including the development of the photographic plate is as described in Example 5.

Example 8 The anodized aluminum plate treated in the manner described in Examples 1 to 4 just up to the point where it is treated with diazo resin, is treated instead with a mixture containing 2 parts by weight of a polyvinyl acetate solution consisting of 60% polyvinyl acetate in methanol (DuPont Elvacet 60-05), 6 parts by weight of ethyl alcohol for thinning since the polyvinyl solution is very thick, and one part by weight of 18% ammonium bichromate. The treatment thereafter including the development of the photographic plate is as described in Example 5.

Although the photographic plate of the present invention is particularly applicable to the manufacture of lithographic printing plates, it has other uses, where it is desired to produce photographically representations on aluminum base surfaces. For example, the photographic plates of the present invention may be used advantageously in the manufacture of plates containing photographically reproduced pictures, insignias, designs or other markings for decorative, advertising or display purposes. In that case, where, for example, the substrate material is a dye, the contrasts and color effects produced thereby could have decorative value.

While the invention has been described with particular reference to specific embodiments, it is to be understood that it is not to be limited thereto but is to be construed broadly and restricted solely by the scope of the appended claims.

What is claimed is:

1. A sensitized photographic plate comprising an aluminum sheet, which has been treated to produce an artificial porous aluminum oxide coating therein, and which has not been completely sealed, so that it retains residual porosity, a substrate in the pores of said coating only partially filling the pores and leaving spaces in said pores, and a layer of light-sensitive material over said coating and over said substrate penetrating into said spaces for enchorage by said porous coating and having one solubility in relation to a solvent in a state before exposure to light and another solubility in relation to said solvent in another state after exposure to light, said light-sensitive material being soluble in said solvent in one of said states and being insoluble in said solvent and in water, hydrophobic and organophilic in its other state, said substrate being of such thickness and in such form as to permit the light-sensitive material to contact said coating and to reach into said spaces so as to be anchored thereby.

2. A sensitized photographic plate as described in claim 1, wherein the substrate material constitutes a compound of the class consisting of ammonium bichromate, sodium bichromate, potassium bichromate, ferric ammonium oxalate, and water-soluble dyes capable of dyeing the aluminum oxide coating.

3. A sensitized photographic plate as described in claim 1, wherein the light-sensitive material is a diazo resin.

4. A sensitized photographic plate as described in claim 1, wherein said light-sensitive material is a film-forming polyvinyl resin of the class consisting of polyvinyl alcohol, polyvinyl acetate and mixture thereof, said resin containing a light-sensitizing agent which insolubilizes the resin under the action of light, said light-sensitive layer having the properties of being hydrophobic and organophili-c after exposure to light.

5. A sensitized photographic printing plate as described in claim 1, in which the aluminum oxide coating constitutes an anodized layer.

'6. A sensitized photographic plate comprising an aluminum sheet which has been treated to produce an artificial porous aluminum oxide coating thereon and which has not been completely sealed, so that it retains residual porosity, a substrate material in the pores of said coating of the class consisting of ammonium bichr-omate, sodium bichromate, potassium bichromate, ferric ammonium oxalate, and water soluble dyes capable of dyeing the aluminum oxide coating, said substrate material only partially filling the pores and providing spaces in said pores, and a layer of diazo resin over said pore-filled coating penetrating into said spaces and firmly anchored thereby to said aluminum oxide coating.

7. A sensitized photographic plate comprising an aluminum sheet which has been treated to produce an artificial porous aluminum oxide coating thereon and which has not been completely sealed, so that it retains residual porosity, a substrate material in the pores of said coating of the class consisting of ammonium bichromate, sodium bichromate, potassium bichr-omate, ferric ammonium oxalate and water soluble dyes capable of dyeing the aluminum oxide, said filler material only partially filling the pores and providing spaces in said pores, and a lightsensitive layer over said pore filled coating penetrating into said spaces and firmly anchored thereby to said aluminum oxide coating, said light-sensitive material being a filmf-orming polyvinyl resin of the class consisting of polyvinyl alcohol, polyvinyl acetate and mixture thereof and containing a light-sensitizing agent which insolubilizes the resin under the action of ultra-violet light, said lightsensitive layer having the properties of being hydrophobic and organophilic after exposure to ultra-violet light.

8. A photographic plate comprising an aluminum sheet which has been treated to produce an artificial porous aluminum oxide coating thereon, and which has not been completely sealed, so that it retains residual porosity, a substrate material in the pores of said coating only partially filling the pores and leaving spaces in said pores, said filler material having hydrophilic and organophobic properties, and a water-insoluble, image-defining layer over said pore-filled aluminum oxide coating having the properties of being Water-insoluble, hydrophobic and organophilic, said aluminum oxide coating containing said substrate material being exposed in the non-image areas of the plate between the areas of the image-defining coating, said substrate being of such thickness and in such form as to permit the image-defining layer to reach into said spaces for anchorage thereby.

9. A photographic plate as described in claim 8, wherein said substrate material constitutes a compound 01 the class consisting of ammonium bichromate, sodium bichromate, potassium bichromate, ferric ammonium oxa late and water-soluble dyes capable of dyeing the aluminum oxide coating by precipitation of coloring matter in said pores.

.10. A photographic plate as dscribed in claim 8, wherein said image-defining layer is a light-exposed diazo resin 11. A photographic plate as described in claim 8, wherein said image-defining layer is a photogra-phically developed mixture comprising before being light-exposed a polyvinyl resin of the class consisting of polyvinyl alcohol, polyvinyl acetate and a mixture thereof, and a light-sensitizing agent.

1-2. A photographic plate as described in claim 9, wherein said image-defining layer is a light-exposed diazo resin.

I13. A photographic plate as described in claim 9, wherein said image-defining layer is a photographically developed mixture comprising before being light-exposed a polyvinyl resin of the class consisting of polyvinyl alcohol, polyvinyl acetate and a mixture thereof, and a light-sensitizing agent.

.14. A photographic printing plate as described in claim 8, in which the aluminum oxide coating constitutes an anodized layer.

15. The process of making a sensitized photographic printing plate comprising the steps of applying to an aluminum sheet having an artificial porous coating of alurninum oxide, which has not been completely sealed, so that it retains residual porosity, a water-solution of a filler material whose molecules are small enough to enter the pores of said coating to cause said pores to be only partially filled when said sheet is dried, and applying over the dried pore-filled coating of aluminum oxide, a lightsensitive layer having one solubility in relation to a solvent in a state before exposure to light and another solubility in relation to s-iad solvent in another state after exposure to light, said light-sensitive material being soluble in said solvent in one of said states and being insoluble in said solvent and in water, hydrophobic and organophil-ic in its other state, said filler material being applied in such amounts and in such form as to leave spaces in the filled pores when the sheet is dried, and said light-sensitive layer extending into .said spaces for anchorage thereby.

16. The process of making a sensitized photographic printing plate as described in claim 15, wherein said lightsensitive layer is a diazo resin.

17. The process of making a sensitized photographic printing plate as described in claim 15, wherein said lightsensitive layer is a polyvinyl film-forming resin of the class consisting of polyvinyl alcohol, polyvinyl acetate and a mixture thereof, and a light-sensitizing agent for insolubilizing said light-sensitive layer upon exposure to ultra-violet light.

References Cited by the Examiner UNITED STATES PATENTS 2,118,053 5/1938 Newsome 148-627 2,504,434 4/1950 Mason 148-62 2,624,673 1/1953 Fr-ost et al 10114*9.2 X 2,710,804 6/ 1955 Schenk et al. 9686 X 2,766,119 10/ 1956 Freedman et al. 9686 3,136,639 6/1964 Deal et a1. 9686 X 3,169,065 2/1965 Sorkin et al. 101149.2 X 3,181,461 5/1965 Fromson 101-l49.2

DAVID KLE P y xam ne UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent Nor, 3, 280, 734 ctober 25, 1966 Howard An Fromson It is hereby certified that error appears in the above numbered patent requiring correction and that the said Letters Patent should read as corrected below.

Column 1, line 31, for "absorb" read adsorb column 4, line 1, for "indicteda" read M indicated line 9, for "portion" read portions H column 6, line 60, for "therein" read thereon line 66, for "enchorage" read anchorage column 8, line 38, for "siad" read said o Signed and sealed this 22nd day of August 1967,

( L) Attest:

ERNEST W. SWIDER EDWARD J. BRENNER Attesting Officer Commissioner of Patents 

8. A PHOTOGRAPHIC PLATE COMPRISING AN ALUMINUM SHEET WHICH HAS BEEN TREADED TO PRODUCE AN ARTIFICIAL POROUS ALUMINUM OXIDE COATING THEREON, AND WHICH HAS NOT BEEN COMPLETELY SEALED SO THAT IT RETAINS RESIDUAL POROSITY, A SUBSTRATE MATERIAL IN THE PORES OF SAID COATING ONLY PARTIALLY FILLING THE PORES AND LEAVING SPACES IN SAID PORES, SAID FILLER MATERIAL HAVING HYDROPHILIC AND ORGANOPHOBIC PROPERTIES, AND A WATER-INSOLUBLE, IMAGE-DEFINING LAYER OVER SAID PORE-FILLED ALUMINUM OXIDE COATING HAVING THE PROPERTIES OF BEING WATER-INSOLUBLE, HYDROPHOBIC AND ORGANOPHILIC, SAID ALUMINUM OXIDE COATING CONTAINING SAID SUBSTRATE MATERIAL BEING EXPOSED IN THE NON-IMAGE AREAS OF THE PLATE BETWEEN THE AREAS OF THE IMAGE-DEFINING COATING, SAID SUBSTRATE BEING OF SUCH THICKNESS AND IN SUCH FORM AS TO PERMIT THE IMAGE-DEFINING LAYER TO REACH INTO SAID SPACES FOR ANCHORAGE THEREBY. 